1. Field of the Invention
The present invention relates to an antenna apparatus and a wireless communication device, and especially, relates to an antenna apparatus using a high-impedance substrate, and a wireless communication device provided with the antenna apparatus.
2. Related Art
An EBG (Electromagnetic Band Gap) is known as a technology for placing a metal plate (a ground plane) in the vicinity of an antenna in order to thin an antenna apparatus (refer to the specification of U.S. Pat. No. 6,262,495 and Japanese Patent No. 3653470). The EBG substrate is constituted such that conductive elements (plate-shaped elements) are arranged in a matrix pattern at a certain height on the metal plate and each of the plate-shaped elements is connected to the metal plate by use of a linear element. This EBG substrate forms an LC parallel resonance circuit by use of a distribution constant circuit, thereby implementing high impedance and suppressing unnecessary current distribution which occurs on the metal plate.
When the EBG substrate is adopted in usage intended to thin the antenna apparatus, important are to place the antenna in the vicinity of the EBG substrate and to thin the EBG substrate itself. As to the thinning of the EBG substrate itself, band characteristics of the EBG substrate are known to be proportional to the thickness of the substrate, and merely a thinning of the substrate leads to narrow-banding, resulting in a lack of a band from a practical standpoint. The thinning, thus, has its own limits. In the EBG substrate described in the specification of U.S. Pat. No. 6,262,495 and Japanese Patent No. 3653470, the EBG substrate becomes thick in, for example, a frequency/band of a cellular phone (about 6 mm or more in 800 MHz, about 2.5 mm or more in 2 GHz), and it is not possible to implement an ultimate thinning of the entire antenna including the EBG substrate viewed from a ground face.
Additionally, there is another problem, which occurs in thinning the EGB substrate, that if the thickness of a high-impedance substrate is becoming thinner while retaining an operational frequency, increased is a size of unit cells periodically arranged in the high-impedance substrate (in other words, the size of each plate-shaped element is increased). A low profile of the antenna requires the unit cells having the number corresponding to the low profile, and thus, an increase in size of the unit cell leads to an enlargement of an area of the substrate.
Further, when the high-impedance substrate is downsized using a dielectric body, there is a problem such as that the band characteristics of the substrate are band-narrowed. Moreover, there is a problem such as that a trouble occurs in curving a surface of the ground plane constituting the high-impedance substrate.